The passive receivers of pseudonoise signals transmitted by the satellites of global positioning systems GPS USA) and GLONASS (Global Navigational Satellite System, Russia) are now widely used and allow the user to accurately determine his coordinates (latitude, longitude, height) and time. The global positioning systems are described in the following references: “Global Navigational Satellite System—GLONASS”. The interface control document. KNIZ VKS, Russia, 1995, and also in “Global Position System. Standard Positioning Service. Signal Specification.” USA, 1993.
The known receivers of a pseudonoise signals (RPNS) operate with a complex digital radio signal consisting of a plurality of signals, transmitted from the satellites which are within the limits of direct visibility, a noise component, as well as components caused by the interference due to repeated reflections of the direct signal from various sites of the earth surface, constructions, etc. This interference causing distortion of the parameters of the signal being received and, as a result, a low accuracy of the receiver characteristics is known as a “multipath” effect.
For detection, tracing and definition of the parameters of the signal being received, the signal is amplified in the RPNS, converted into an intermediate frequency and digitized in the radio-frequency unit of the receiver. After that the digital correlation technique is used for the final detection of the signals. Known in the art are digital correlators performing the correlation of the incoming complex digital radio signal by multiplying its digits by the local copy of the sought signal generated inside the correlator followed by accumulation of the correlation results through a definite time interval. As a rule, this interval is taken equal to 1 millisecond, that is equal to the length of the code sequence/of the GPS/GLONASS code. To close the loop of tracing the frequency (phase) of a signal being received and the delay of the coded use is made of the calculator, which reads out the information from the accumulator and closes the tracing loop by means of its routine. For tracing the code (code delay), the tracing cycle uses the results of correlation of the input signal with the early (advanced) and late (delayed) copies of this signal or with a difference (early-minus-late) copy. In this case, besides the main (direct) signal coming from the satellite there is an additional delayed signal due to the multipath effect that will result in distortion of the discrimination characteristic of the tracing cycle and, as a corollary, appearance of an additional error in the range finding. In addition, since the signal coming with the satellite is rather weak and its amplitude much below the natural thermal noise level, the correlator should ensure the best possible signal-to-noise ratio.
It is also known, that the use of a narrow correlator in the tracing cycle behind the code increases the signal-to-noise ratio and, besides, reduces the negative multipath effect (cf. A. J. Dierendonck, P. Fenton, T. Ford “Theory and Performance of Narrow Correlator Spacing in GPS receiver”, Navigation: Journal of The Institute of Navigation Vol. 39, No. 3, Fall. 1992. See also P. Fenton, A. J. Dierendonck “Pseudorandom noise ranging receiver which compensates for multipath distortion by dynamically adjusting the time delay spacing between early and late correlators”, U.S. Pat. No. 5,390,207, Feb. 14, 1995). However, the narrow correlator only weakens the multipath effect and does not allow one to eliminate this effect completely.
Also known in the art are devices such as a “gating correlator” and an “improved gating correlator”, permitting one to lower the negative effect of the multipath transmission even more (cf. “L. Garin, J-M. Rousseau “Enhanced Strobe Correlator Multipath Rejection for Code and Carrier”, ION-GPS 1997, Session B2). The “gating correlator”, based on a linear combination of two narrow correlators, excluding the multipath effect for a significant amount of the delayed signals, nevertheless, worsens the signal-to-noise ratio by 3 dB relative to the source narrow correlator. The “improved gating correlator” operating in the early strobe field and thereby performs the correlation of the signal part near to front edge eliminating the multipath effect for even a greater amount of the delayed signals but, at the same time, it worsens the signal-to-noise ration by 6 dB relative to the narrow source correlator. The narrow source correlator as mentioned here is one of the two narrow correlators whose combination forms a “gating correlator” and which has a minimum temporary shift relative to the exact copy of the signal being traced.
The most relevant technical solution with respect to the proposed invention are the method and device described in PCT application WO 97/06446 of Feb. 20, 1997 “Multipath error reduction in a spread spectrum receiver for ranging applications”. In this application, in order to form the signal from the discriminator output for tracing code delay, an offer is made to form a differential copy of a pseudonoise signal, consisting of a “sequences of identical non-zero gate pulses formed on the boundary of the characters of a pseudonoise sequence, such that the duration of each gate pulse is less than pulse duration in the pseudonoise code; the gate pulse is positive in the square positive and negative parts and also has positive or negative polarity in the central part that corresponds to the change in the positive or negative side of the pseudonoise sequence characters”. In one of the embodiments described in the cited application, a difference pseudo-random sequence (PRS) is proposed to be formed by generation of four replicas: “an early replica advancing the exact copy by 1/(2k) fraction of the RPS character, a late replica delayed with respect to the exact copy by 1/(2k) by a fraction of the RPS character, an early replica advancing the exact copy by N/(2k) fraction of the character, and one more late replica delayed with respect to the exact RPS by N/(2k) character, where N and k are the whole numbers and N<k. In particular, it is offered to select k=10, N=2. By subtracting from the early copy the corresponding late one, a difference copy is obtained and used in the narrow correlator. Then, by subtracting from one difference copy the second copy, one gets a sequence of gate pulses similar to those described above. Thus, the described method completely corresponds to the method of “gating correlation”, mentioned above and, accordingly, has similar disadvantages. So in the system with four replicas the signal-to-noise ratio will be By N times worse, than for the source narrow correlator with an offset ±1/(2k) of the character. Therefore, for N=2, the loss will be up to 3 dB.